Levoglucosan: A Calorimetric, Thermodynamic, Spectroscopic, and

Article pubs.acs.org/jced

Levoglucosan: A Calorimetric, Thermodynamic, Spectroscopic, and Computational Investigation Inês M. Rocha,† Tiago L. P. Galvaõ ,† Erlin Sapei,‡ Maria D. M. C. Ribeiro da Silva,*,† and Manuel A. V. Ribeiro da Silva† †

Centro de Investigaçaõ em Química, Department of Chemistry and Biochemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, P-4169-007, Portugal ‡ School of Chemical Technology, Department of Biotechnology and Chemical Technology, Chemical Engineering Research Group, Aalto University, POB 16100, FIN-00076 Aalto, Finland S Supporting Information *

ABSTRACT: A comprehensive analysis of the thermochemical properties of levoglucosan, using static bomb combustion calorimetry, Knudsen effusion technique, and differential scanning calorimetry, is presented. The experimental results allow us to derive the enthalpy of formation, in the gaseous phase, and thereafter to do a comparison with the same parameter obtained computationally. The good agreement between the experimental and computational results gives confidence to our determinations, particularly when they are compared with others already reported in literature. After testing the computational methodology, the ionization energy, electron affinity, proton affinity, gas-phase basicity, gas-phase acidity, and bond dissociation enthalpies of levoglucosan were also obtained. The presence of intramolecular hydrogen bonds in the most stable conformation of levoglucosan was verified by applying Quantum Theory of Atoms in Molecules calculations. Furthermore, a joint differential scanning calorimetry and temperature dynamic Fourier transform infrared (FT-IR) spectroscopic study was used to study the crystalline phase of levoglucosan between 298.15 K and the melting temperature.

1. INTRODUCTION

The static bomb combustion calorimetry was used on the measurement of the standard massic energy of combustion of levoglucosan, which allowed to derive the corresponding standard (p° = 0.1 MPa) molar enthalpy of formation, in the crystalline phase, at T = 298.15 K. The vapor pressures were measured, at several temperatures, by Knudsen effusion technique, and by application of the Clausius−Clapeyron equation to the results obtained, the molar enthalpy and entropy of sublimation of levoglucosan were derived. The values of the standard molar enthalpy of formation, in the crystalline phase, and the standard molar enthalpy of sublimation were combined to derive the standard molar enthalpy of formation, in the gaseous phase, at T = 298.15 K. This value was compared with the same property obtained computationally from theoretical calculations, using G3 and G3(MP2) computational methods. The computational study of levoglucosan was extended to the ionization energy, electron affinity, proton affinity, gas-phase basicity, gas-phase acidity, and bond dissociation enthalpies. Electron density calculation using the

Pyrolysis of cellulose biomass is one of the most promising techniques for bio-oil production to substitute the fuels. The composition of the pyrolysis products is very complex and represents a variety of biopolymers derivatives and extractive compounds.1,2 Levoglucosan (1,6-anhydro-β-D-glucopyranose) is the major intermediate component in the pyrolysis of cellulose that controls char formation.3,4 Hosoya et al.5 studied the pyrolytic reaction pathways and the pyrolysis behavior of levoglucosan in the vapor and liquid/solid phases. Thermochemical properties, such as vapor pressure, enthalpy of combustion, and enthalpy of formation, in the gaseous phase, of levoglucosan, can play an important role in the simulation of process design/process development dealing with the pyrolytic reactions, involving this compound, that occur in the biorefinery. The standard molar enthalpy of combustion6,7 and vapor pressures4,8 have been previously reported. However, discrepancies between the results obtained in those studies, which will be evaluated in section 4 (Results and Discussion), together with the importance of the compound, lead us to take a comprehensive thermochemical study using static bomb combustion calorimetry, Knudsen effusion technique, and differential scanning calorimetry. © XXXX American Chemical Society

Received: March 1, 2013 Accepted: April 3, 2013

A

dx.doi.org/10.1021/je400207t | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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The calorimetric system was previously calibrated, according to the procedure suggested by Coops et al.,14 by the combustion of benzoic acid (NIST Standard Reference Material 39j), with a massic energy of combustion, under certificate conditions, of Δcu= −(26434 ± 3) J·g−1.9 From six calibration experiments, the energy equivalent of the calorimeter obtained was ε(calor) = (15905.0 ± 0.8) J·K−1 (quoted uncertainty refers to the standard deviation of the mean), for an average mass of water added to the calorimeter of 3119.6 g. Calorimeter temperatures were measured with an uncertainty of ± (1·10−4) K, at time intervals of 10 s, using a quartz thermometer (Hewlett-Packard HP 2804A) interfaced to a computer programmed to calculate the adiabatic temperature change, by means of the LABTERMO program.15,16 At least 100 readings were taken before the ignition of the samples, which were made at (298.150 ± 0.001) K by the discharge of capacitor (1400 μF) through a platinum ignition wire (ϕ = 0.05 mm, Goodfellow, mass fraction 0.9999). The cotton thread fuse (empirical formula CH1.686O0.843) has a standard massic energy of combustion assigned to Δcu° = −16240 J·g−1,14 a value previously confirmed in our laboratory. After the ignition, 100 readings were taken for the main period, followed by another 100 readings for the final period. The electrical energy for the ignition, ΔUign, was determined from the charge in potential difference across the capacitor when discharged through the platinum ignition wire. The energy ° (HNO3, corrections for the HNO3 formation was based on ΔfUm aq, 0.1 mol·dm−3) = −59.7 kJ·mol−1.17 All of the necessary weighings for the combustion experiments were made in a Mettler Toledo 245 balance with a sensitivity of ± 10−5 g. An estimated pressure coefficient of massic energy, (∂u/∂p)T = −0.2 J·g−1·MPa−1, at T = 298.15 K, a typical value for the most organic compounds,18 was assumed. The amount of levoglucosan used in each experiment was determined from the total mass of carbon dioxide (Mettler Toledo AT 201 balance, sensitivity (± 1·10−5) g, produced deducing that formed from cotton-thread fuse and of Melinex. The standard state corrections, ΔU∑, and the heat capacities of the bomb contents, εi and εf, were calculated by the procedure given by Hubbard et al.19 using the enthalpies of solution of CO2 and O2 in pure water reported by Hu et al.20 The specific density used to calculate the true mass from the apparent mass in air was 1.621 g·cm−3, determined by X-ray measurements.21 2.2. Differential Scanning Calorimetry (DSC) Measurements. The temperatures and the standard molar enthalpies of phase transitions of levoglucosan were measured in a power compensation differential scanning calorimeter, model SETARAM DSC 141. The temperature and the power scale were calibrated by measuring, respectively, the melting temperature and enthalpy of the following reference materials:22 o-terphenyl (CAS 84-15-1), benzoic acid (CAS 65-85-0), indium (CAS 7440-74-6), triphenylene (CAS 217-59-4), tin (CAS 7440-31-5), perylene (CAS 198-55-0), lead (CAS 7439-92-1), and zinc (CAS 7440-66-6). The sublimated samples of levoglucosan sealed in aluminum crucibles were heated at a heating rate of 3.3·10−2 K·s−1; four experiments leading to consistent results were

Quantum Theory of Atoms in Molecules was applied to verify the presence of intramolecular hydrogen bonds. Differential scanning calorimetry was used to obtain the enthalpy of fusion of levoglucosan and to detect any other phase transitions between 298.15 K and the melting temperature. A structure analysis was performed using Fourier transform infrared (FT-IR) experiments at several temperatures.

2. EXPERIMENTAL SECTION 2.1. Materials and Purity Control. The supplier and purification details of levoglusocan (Figure 1) are presented in

Figure 1. Molecular structure of levoglucosan.

Table 1. The purity of the samples was checked by gas chromatograph using an Agilent HP-4890 with a HP-5 column, 5 % diphenyl and 95 % dimethylpolysiloxane, using nitrogen as carrier gas. Even though the initial purity of levoglucosan was high, the samples were sublimed prior to measurement to remove the adsorbed water since this compound is hygroscopic. To ensure that levoglucosan was burned dry, all of the procedures for the sample preparation were carried out under nitrogen atmosphere. The average ratio of the mass of carbon dioxide recovered in the combustion experiments to those calculated from the mass of samples used in each experiment, together with the uncertainties (twice the standard deviation of the mean), was (1.0001 ± 0.0008). The benzoic acid (CAS 65-85-0) NIST Standard Reference Material, sample 39j,9 was used to calibrate the static bomb calorimeter. The relative atomic masses used in the calculation of all molar quantities throughout this paper were those recommended by the IUPAC commission in 2009,10 yielding 162.1406 g·mol−1 for the molar mass of levoglucosan. 2.2. Static-Bomb Combustion Calorimetry Measurements. The standard (po = 0.1 MPa) massic energy of combustion of levoglucosan was determined using an isoperibol static bomb calorimeter previously described.11,12 All of the combustion experiments were performed in a stainless steel twin valve bomb (type 1105, Parr Instrument Company, with internal volume of 0.340 dm3) having inside 1.00 cm3 of deionized water. The samples were handled under nitrogen atmosphere and sealed in polyester bags made of Melinex, using the technique described by Skinner and Snelson,13 which proposed a massic energy of combustion of dry Melinex of Δcu° = −(22902 ± 5) J·g−1, a value previously confirmed in our laboratory. The mass of Melinex used in each experiment was corrected for the mass fraction of water (0.0032).13 The bomb was purged twice to remove the air before being charged with oxygen at p = 3.04 MPa. Table 1. Supplier and Purification Details

a

chemical name

CAS

supplier

initial purity

purification methoda

final purity (mass fraction)

analysis methodb

levoglucosan

498-07-7

Carbosynth

min 98 % (HPLC)

sublimation

0.9993

GC

Details in text. bGas chromatography. B



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Table 2. Derived Standard (p° = 0.1 MPa) Molar Energy of Combustion, ΔcUm ° , Standard Molar Enthalpy of Combustion, ΔcHm °, and Standard Molar Enthalpy of Formation, in the Condensed Phase, ΔfH°m(cr) of Levoglucosan, at T = 298.15 K −⟨Δcu°⟩(cr)

−ΔcUm ° (cr)

−ΔcHm ° (cr)

−ΔfHm ° (cr)

levoglucosan

J·g−1

kJ·mol−1

kJ·mol−1

kJ·mol−1

this work Skuratov et al.7 Karrer et al.6

17461.0 ± 5.5

2831.1 ± 1.9

2831.1 ± 1.9 2831.7 ± 0.9 2839.8

959.1 ± 2.1

performed, using the same experimental procedure as in the calibration runs. 2.3. FT-IR Measurements. The FT-IR spectra of levoglucosan was recorded with a Spectrum BX FT-IR, PerkinElmer spectrometer, in transmission mode, using a GladiATR (Pike technologies) with a heated diamond crystal plate coupled to a digital temperature controller module. 2.4. Mass-Loss Knudsen Effusion Measurements. The vapor pressures of levoglucosan, in the range of (0.1 to 1.0) Pa, were measured, at several temperatures, by Knudsen effusion technique. The apparatus and experimental procedure described by Ribeiro da Silva et al.23 were used. This apparatus enables the simultaneous operation of nine effusion cells, at three different temperatures. Determining the mass, Δm, sublimed from the effusion cell during a time period, t, by weighing the cell to ± 0.01 mg in a Mettler AE 163 balance, before and after the measurement, was possible to calculate the vapor pressure, p, at the temperature, T, using the Knudsen eq 1, where Ao is the area of the effusion orifice, R is the gas constant (R = 8.314472 J·K−1·mol−1), M is the molar mass of the effusion vapor, and wo is the Clausing probability factor. The areas and Clausing factors of the effusion orifices, in the platinum foil of 0.0125 mm thickness, are presented in the Supporting Information, Table S1. p=

Δm ⎛⎜ 2πRT ⎞⎟1/2 · Aowot ⎝ M ⎠

AOH + H+ → AOH 2+

PA = −Δr Hm°

ΔbasGm° = −Δr Gm° AOH → AO− + H+

(4)

Δacid Hm° = −ΔrHm°

Δacid Gm° = −Δr Gm° AOH → AO• + H

(5)

D = −Δr Hm°

(6)

Electron density analysis in the framework of the Quantum Theory of Atoms in Molecules (QTAIM)28,29 was performed using the AIMAll program package,30 from the optimized structures, at the MP2(full)/6-31G(d) level of theory, during the G3(MP2) calculations.

4. RESULTS AND DISCUSSION 4.1. Experimental Enthalpy of Formation, in the Crystalline Phase. Detailed results of each combustion experiment performed for levoglucosan are presented in Table S2 in the Supporting Information. The internal energy of the isothermal bomb process, ΔU (IBP), is calculated through eq 7, where ΔTad is the adiabatic temperature rise and εf is the energy of the bomb contents after ignition. The energy equivalent, ε(calor), was corrected for the deviation of the mass of water used from 3119.6 g, Δm (H2O), in each combustion experiment.

(1)

ΔU (IBP) = − {ε(calor) + Δm(H 2O)cp(H 2O, l) + εf }Δ

3. COMPUTATIONAL DETAILS The computational calculations of the gaseous phase thermodynamic properties of levoglucosan were performed using the Gaussian 03 software package.24 Standard molar enthalpies and Gibbs energies, at T = 298.15 K, used to estimate different thermodynamic properties, were calculated computationally using Gaussian-3 (G3)25 and Gaussian-3 with MP2 single point energies for basis set extrapolation (G3(MP2))26 composite methods. The ionization energies and electron affinities obtained in this work, at T = 298.15 K, with G3(MP2), are adiabatic and follow the Ion Convention for treating the thermochemistry of the electron, as recommended by Lias and Bartmess.27 The standard molar ionization energies (IE), electron affinities (EA), proton affinities (PA), gas-phase basicities (ΔbasG°m), gasphase acidities (ΔacidH°m and ΔacidG°m), and bond dissociation enthalpies (D) were calculated, at T = 298.15 K, as recommended,27 where A is levoglucosan, tetrahydropyran, or phenol, which were used as references to test the computational methodology, using eqs 2 to 6. AOH → AOH+ + e−

IE = Δr Hm°

(2)

AOH + e− → AOH−

EA = −Δr Hm°

(3)

Tad + ΔUign

(7)

The individual values of the standard mass energies of combustion for all experiments of levoglucosan are referred to eq 8, for which the mean value, ⟨Δcu°⟩, the derived standard molar energy and enthalpy of combustion and the standard molar enthalpy of formation, in the condensed phase, at T = 298.15 K, are listed in Table 2. C6H10O5(cr) + 6O2 (g) → 6CO2 (g) + 5H 2O(l)

(8) 31,32

According to normal thermochemical practices, the uncertainties assigned to the standard molar energy and enthalpy of combustion are twice the overall standard deviation of the mean and include the uncertainties of the calibration and of the auxiliary quantities used. To derive ΔfH°m(cr) from ΔcH°m(cr), the standard molar enthalpies of formation of CO2 (g) and H2O (l), at T = 298.15 K, −(393.51 ± 0.13) kJ·mol−1,33 and −(285.830 ± 0.042) kJ·mol−1,33 respectively, were used. Two values of the standard molar enthalpy of combustion for levoglucosan are described in the literature and presented in Table 2. Karrer et al.6 obtained, in 1921, a lower value differing by 8.7 kJ·mol−1 from the value obtained in this work. The combustion calorimetry of this compound was repeated by Skuratov et al.,7 who obtained a similar value to that value determined in the present work. However, both previous C



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Figure 2. DSC thermogram of the levoglucosan with phase transitions I and II.

works6,7 did not describe the bomb conditions used on the combustion measurements nor any special treatment for the hygroscopicity of levoglucosan. On the other hand, the standard massic energy of combustion reported on those works did not describe the use of the energetic corrections to standard state (po = 0.1 MPa).18 4.2. Thermodynamic Stability of the Crystalline Phase. Four DSC measurements were performed in a temperature range between 353 K and 463 K, and two phase transitions were observed (Figure 2). The first phase transition appears at T = (384.9 ± 0.3) K followed by a second phase transition at T = (455.4 ± 0.1) K, corresponding to the melting of levoglucosan. After cooling, the sample was reheated, and the first phase transition reappeared at same temperature. Table 3 summaries the temperatures, the standard molar enthalpies, and entropies of both phase transitions obtained in this work, as well as the values determined by Shafizadeh.34

Figure 3. FT-IR spectra in the OH stretching region of the levoglucosan at different temperatures.

Table 3. Temperatures of Transition, Ttrans, and the Standard Molar Enthalpies, ΔtransHm ° (Ttrans), and Entropies, ΔtransSm ° (Ttrans,ptrans), of the Transition of Levoglucosan levoglucosan I II

this work Shafizadeh34 this work Shafizadeh34

Ttrans

ΔtransHm ° (Ttrans)

ΔtransSm ° (Ttrans, ptrans)

K

kJ·mol−1

J·K−1·mol−1

384.9 ± 0.3 385 455.4 ± 0.1 453

23.2 ± 0.4 24.94 3.3 ± 0.2 3.4

60.4 ± 1.0 64.9 7.3 ± 0.4 7.5

spectra are similar, presenting three peaks with a wavenumber in the range of 3000 cm−1 to 3600 cm−1. The main difference occurs when the temperature is increased to 418 K, after the first phase transition, since instead the three peaks, it appears one broad peak at higher wavenumbers. Above the melting temperature (470 K), the spectrum of levoglucosan is identical to the one at T = 418 K. This behavior is in agreement with the thermodynamic study, since to the first phase transition corresponds also to higher enthalpy and higher entropy than for the second transition. The vapor pressure study was performed using the mass-loss Knudsen effusion technique in a temperature interval lower than the temperature of the first phase transition. The standard molar enthalpy of sublimation at the mean temperature of the experimental temperature range, ΔcrgHm° (⟨T⟩), was derived from the integrated form of the Clausius−Clapeyron equation, ln(p/Pa) = a − b·(T/K)−1, where a is a constant and b = ΔgcrHm ° (⟨T⟩)/R. Detailed results of the vapor pressure measurements are presented in Table S3 of the Supporting Information. Combining the vapor pressures of levoglucosan at several temperatures for the three different orificies, the following equation was obtained:

Several studies4,34,35 defined the first phase transition as the change of the crystal to a plastic crystal. In a plastic crystal, the molecules are more weakly bonded, and they move more freely, so the phase transition from crystal to plastic crystal appears before the complete disruption of the crystal lattice by fusion at a higher temperature. In this case, the entropy of the transition from the crystal phase to a plastic crystal is higher than the entropy of the transition from the plastic crystal to the liquid phase (Table 3). The experimental values, obtained in this work, are in good agreement with those already presented in the literature,34 without an associate uncertainty. Only the obtained standard molar entropy of transition I is lower than the value presented by Shafizadeh.34 In Figure 3, the FT-IR spectra of O−H stretching of levoglucosan is presented at four different temperatures. At the temperatures lower than the plastic crystal phase, until 358 K, the

ln(p) = (40.99 ± 0.59) − D

15824 ± 193 T

(9)



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study, a difference in the enthalpic and entropic terms is observed: in the present work, the enthalpy and entropy of sublimation is slightly higher than the Oja and Suuberg values (c.f. Table 5). 4.4. Conformational Analysis. The molecular structures, relative Gibbs energies, and molar percentages of the various conformations of levoglucosan, computed at the G3(MP2) level of theory, are presented in Figure 2. The molar percentages were calculated according to eq 11, where G°m(i) is the computed standard molar Gibbs energy of each conformer, at T = 298.15 K, and is presented in table S4 of the Supporting Information.

Table 4 reports the vapor pressure at the mean temperature, p(⟨T⟩), the standard molar enthalpy, and entropy of sublimation, Table 4. Standard (po = 0.1 MPa) Molar Enthalpy, ΔgcrHm ° , and Entropy, ΔgcrSm ° of Sublimation and the Pressure, at the Mean Temperature of the Experiments, ⟨T⟩, for Levoglucosan ⟨T⟩ levoglucosan this work Booth et al.8 Oja and Suuberg4

p(⟨T⟩)

K

Pa

370.19 308 364.25

1.71·10−1 1.15·10−4 6.34·10−2

ΔgcrHm ° (⟨T⟩)

ΔgcrSm ° (⟨T⟩,p(⟨T⟩))

−1

J·K−1·mol−1

KJ·mol

131.0 ± 1.7 68.8 ± 4.4 125.1 ± 1.0

353.9 ± 4.6 223.4 ± 14.3 343.4 ± 2.7

xi =

respectively, ΔgcrH°m(⟨T⟩) and ΔgcrS°m(⟨T⟩, p(⟨T⟩)), obtained in this work. Previous studies performed by other researchers by mass-loss Knudsen effusion technique coupled to a thermogravimetry (TGA) technique4 and Knudsen effusion mass spectrometry8 are also presented in Table 4. Booth et al.8 studied the vapor pressures in a range of temperatures before the first phase transition, while Oja and Suuberg4 measured the vapor pressures before and after the first phase transition from crystal to plastic crystal. The difference between the ΔgcrH°m(⟨T⟩) of both phase transitions [ ΔgcrH°m(cr, ⟨Tcr⟩) = 125.1 ± 1.0 kJ·mol−1;4 ΔgcrHm ° (plast. cr, ⟨Tplast.cr⟩) = 100.3 ± 5.9 kJ·mol−1;4 ΔgcrHm ° (cr, ⟨Tcr⟩) − ΔgcrHm ° (plast. cr, ⟨Tplast.cr⟩) = (24.8 ± 6.0) kJ·mol−1] is in agreement with the enthalpy of phase transition from crystal to the plastic crystal, obtained by DSC measurements in this work [(23.2 ± 0.4) kJ·mol−1]. The standard molar enthalpy of sublimation, at T = 298.15K, was derived using eq 10, where a value for the difference between the molar heat capacity of the gas and the molar heat capacity of the solid, Δcrg Cp,m ° (⟨T⟩), already used for other organic compounds,36−40 of −50 J·K−1·mol−1,41 was considered.

exp{ −[Gm° (i)/RT ]} ·100 n ∑i = 1 exp{ −[Gm° (i)/RT ]}

(11)

The chair conformation of the molecule and the axial orientation of the hydroxyl groups were chosen in accordance to the crystal structures.21,42−44 A boat conformation would result in higher strain energy of the cyclic structure, and an equatorial orientation of the three adjacent hydroxyl groups would result in higher electrostatic repulsion between the oxygen atoms of the hydroxyl groups. The different conformations of levoglucosan, obtained by changing the direction of the hydroxyl groups, are presented in Figure 4.

Δcrg Hmo(298.15 K) = Δcrg Hmo(⟨T ⟩) + Δcrg C po,m (298.15 − ⟨T ⟩)

(10)

The standard molar enthalpies, entropies, and Gibbs energies of sublimation, as well as, the vapor pressure of levoglucosan, at T = 298.15 K, determined in this work are listed in Table 5. Booth Figure 4. Molecular structures, relative standard molar Gibbs energies, at T = 298.15 K, and molar percentages of the lowest-energy conformers of levoglucosan, optimized at the MP2(Full)/6-31G(d) level of theory.

Table 5. Standard (po = 0.1 MPa) Molar Enthalpy, ΔgcrHm °, Entropy, ΔgcrS°m, and Gibbs Energy, ΔgcrG°m, of Sublimation and Pressure, p, at T = 298.15 K, for Levoglucosan

levoglucosan this work Booth et al.a Oja and Suubergb

p(298.15 K)

ΔgcrH°m

ΔgcrS°m

ΔgcrG°m

Pa

kJ·mol−1

J·K−1·mol−1

kJ·mol−1

134.6 ± 1.7 69.3 ± 4.4 128.4 ± 1.0

254.3 ± 4.6 53.9 ± 14.3 234.7 ± 2.0

58.8 ± 2.2 53.9 ± 6.3 58.4 ± 1.2

−6

4.8·10 4.8·10−5 5.9·10−6

The stability of the conformers can be a result of a compromise between electrostatic repulsions and O−H···O hydrogen bonds. QTAIM analysis is used to identify hydrogen bonds in the most stable conformation of levoglucosan (conformation I, Figure 4), which can be useful to understand other thermodynamic properties. From the QTAIM analysis two bond critical points were obtained, between the hydrogen atom in position 12 and the oxygen atom in position 13 (H12···O13) and between the hydrogen atom in position 16 and the oxygen atom in position 17 (H16···O17), which indicate the presence of two intramolecular noncovalent bonds. Table 6 presents, for each bond critical point, the electron density (ρ), which has been correlated with binding energies for different types of interactions;45−47 the Laplacian of the electron density (Δρ), which indicates if the electron density is more concentrated in the interatomic region (Δρ < 0) or at the individual atoms (Δρ > 0) along the bond path;29 and the total electron energy density (H), which is more negative for

a

Derived from the values at the mean temperature of ref 8. bDerived from the values at the mean temperature of ref 4.

et al.8 and Oja and Suuberg4 data, reported at the mean temperature of the experimental temperature range of each work, was also corrected to 298.15 K using the same ΔgcrCp,m ° value and are presented in Table 5. Booth et al. data8 is very different from the obtained values in this work and in Oja and Suuberg.4 Besides the similarity of the Gibbs energies obtained in this work and in the Oja and Suuberg E



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G3 approach, which are more computationally expensive, only the enthalpy of the most stable conformer was used. In the G3(MP2) calculations, the difference between the enthalpy of the most stable conformer and the Boltzmann weighted mean of every conformer was 0.9 kJ·mol−1, and, consequently, no significant error in the estimation of the enthalpy of formation, in the gaseous phase, is expected by only considering the most stable conformer in the G3 calculations. The values obtained through the atomization reactions, using both methods, are not in agreement with the experimental value, considering both the chemical accuracy (4 kJ·mol−1) and the experimental uncertainty of the enthalpy of formation, in the gaseous phase, of the compound. In order to minimize the shortcomings of the computational methods through the cancellation of errors, reactions I and II (Figure 5) were selected, which match the hybridization states of the atoms in the reactants and products, and present more similar strain energies and possible cyclic delocalization. In these reactions, the chair conformation of the cyclohexane derivatives used in the reactants and products was preferred, which is more stable than the boat conformation.52,53 The methyl and hydroxyl groups of methylcyclohexane and hydroxylcyclohexane, respectively, are in equatorial positions, since, when the functional groups occupy the axial positions, the conformation becomes enthalpically unfavorable due to more stereochemical hindrance of the methyl or hydroxyl groups with the two axial hydrogen atoms,54,55 whereas the methoxy group in tetrahydro-2-methoxypyran exists preferably in the axial position due the anomeric effect between one lone pair of electrons of the oxygen atom of the ring and the σ (C−O) antibonding orbital of the methoxy group.56−58 The methyl moiety of the methoxy group is also in gauche to the adjacent C−O bond of the ring because of another anomeric effect, between one of the lone pairs of electrons of the oxygen atom of the methoxy group and the σ (C−O) antibonding orbital of the ring.57 The values of the enthalpy of formation of levoglucosan, in the gaseous phase, obtained from G3(MP2) and G3 computational calculations, using each of the working reactions presented in Figure 5, are reported in Table 7. The theoretical values are in agreement with experiment using both methods and for both reactions, considering the experimental uncertainty and the chemical accuracy (4 kJ·mol−1). Reaction II, which considers more similar bonds in reactants and products by considering tetrahydro-2-methoxypyran, gives better results. Since there seems to be no advantage in using the computationally more expensive G3 calculations in reactions whose electron effects are sufficiently well balanced in both reactants and products, the G3(MP2) composite method was chosen to obtain other thermodynamic properties of levoglucosan.

Table 6. QTAIM Bond Critical Properties for Intramolecular Hydrogen Bonds of the Most Stable Conformation of Levoglucosan ρ

Δρ

H

interaction

au

au

au

H12···O13 H16···O17

0.016 0.029

0.057 0.094

−0.00004 −0.00194

interactions with significant sharing of electrons.48 The H16···O17 and H12···O13 intramolecular interactions are indeed hydrogen bonds according to the ranges obtained by Nakanishi et al.,49 since the values of the electron density, its Laplacian, and the total electron energy density are between 0.01 and 0.04 au, 0.04 and 0.12 au, and −0.004 and 0.002 au, respectively, typical values of intramolecular hydrogen bonds.49 The H16···O17 intramolecular hydrogen bond is stronger than the H12···O13 intramolecular hydrogen bond, since the density is higher and the total electron energy density is more negative,49 considering that the total electron energy density at the bond critical point is a more appropriate index than the Laplacian of the electron density to analyze weak interactions in terms of energy.28,29,50,51 4.5. Experimental and Computational Enthalpies of Formation, in the Gaseous Phase. The experimental and computational standard (po = 0.1 MPa) molar enthalpy of formation, in the gaseous phase, at T = 298.15 K, is presented in Table 7. The computational enthalpies of formation were Table 7. Experimental and Computational Standard Molar Enthalpy of Formation, in the Gaseous Phase, of Levoglucosan G3 (MP2) ΔfHm ° (g) reaction

kJ·mol−1

atomization I II

−835.7 −828.2 −823.8

Δ

G3 ΔfHm ° (g)

a

−1

kJ·mol

11.2 3.7 −0.7

−1

kJ·mol

−847.5 −831.0 −825.1

Δa kJ·mol 23.0 6.5 0.6

ΔfHm ° (g)b −1

kJ·mol−1 −824.5 ± 2.7

a

Diference between the experimental and computational values. Obtained, in this work, combining the enthalpy of formation, in the crystalline phase (Table 2) and the enthalpy of sublimation (Table 5).

b

calculated considering the enthalpies of formation for auxiliary compounds presented in Tables S4 and S5 of the Supporting Information. In the G3(MP2) calculations, the molar percentages of all conformers were considered in order to obtain the standard molar theoretical enthalpy, at T = 298.15 K, and in the

Figure 5. Working reactions used to obtain the enthalpy of formation, in the gaseous phase, of levoglucosan, from theoretical calculations. F



Article

Table 8. Experimental and G3(MP2) Computed Standard Molar Ionization Energy, IE, Proton Affinity, PA, Gas-Phase Basicity, ΔbasG°m, Gas-Phase Acidity, ΔacidH°m, Bond Dissociation Enthalpy, D, of Tetrahydropyran and Phenol, at T = 298.15 Ka IE/kJ·mol−1

PA/kJ·mol−1

ΔbasG°m/kJ·mol−1

ΔacidH°m/kJ·mol−1 exp.

compound

exp.

G3(MP2)

exp.

G3(MP2)

exp.

tetrahydropyran

905.0 ± 4.859

907.4 (−2.4)

822.1 ± 8.060

829.1 (−7.0)

794.7 ± 8.060

G3(MP2)

G3(MP2)

exp.

G3(MP2)

799.0) (−4.3) 1454 ± 8.061

phenol

1460 ± 8.0

62

1465 ± 8.063 a

D(O−H)/kJ·mol−1

1462.2 (−8.2) 1462.2 (−2.2) 1462.2 (+2.8)

371.3 ± 2.364 368.2 ± 6.3

65

374.8 (−3.5) 374.8 (−6.6)

The difference between the experimental and computational values is indicated in brackets.

Table 9. G3(MP2) Computed Standard Molar Ionization Energy, IE, Electronic Affinity, EA, Proton Affinities, PA, Gas-Phase Basicities, ΔbasGm ° , Gas-Phase Acidities, ΔacidHm ° and ΔacidGm ° , Bond Dissociation Enthalpies, D, of Levoglucosan, at T = 298.15 Ka IE

EA

PA

ΔbasGm °

ΔacidHm °

ΔacidGm °

D(O−H)

kJ·mol−1

kJ·mol−1

kJ·mol−1

kJ·mol−1

kJ·mol−1

kJ·mol−1

kJ·mol−1

847.2

−170.6

803.9 (O17) 818.2 (O21)

771.2 (O17) 787.1 (O21)

1481.1 (H12) 1479.0 (H14) 1513.8 (H16)

1456.7 (O11−H12) 1454.3 (O13−H14) 1485.7 (O15−H16)

463.3 (O11−H12) 445.3 (O13−H14) 448.1 (O15−H16)

Brackets indicate the site where the proton is bonded (PA and ΔbasG) and the dissociated bond (ΔacidH, ΔacidG and D); bold text indicates the more thermodynamically favorable value.

a

4.6. Gas-Phase Ion Energetic Properties. Before obtaining other thermodynamic properties of levoglucosan, in the gaseous phase, G3(MP2) was tested for the standard molar ionization energy,59 proton affinity60 the gas-phase basicity60 of tetrahydropyran, and the gas-phase acidity61−63 and O−H bond dissociation enthalpy of phenol.64,65 The computational results obtained in this work, presented in Table 8, are in agreement with literature values within the chemical accuracy (4 kJ·mol−1). Therefore, the ionization energy, electron affinity, proton affinity, gas-phase basicity, gas-phase acidity, and bond dissociation enthalpies were obtained computationally for levoglucosan at the G3(MP2) level of theory and are presented in Table 9. From the proton affinities and gas-phase basicities results, it is possible to verify that the proton is bonded more favorably to the oxygen in position 21 rather than the oxygen in position 17, which may be due to the oxygen in position 17 being the acceptor atom of an intramolecular hydrogen bond from O15−H16 which may be destabilized by the bonding of a proton at that site of the molecule. The enthalpies and Gibbs energies of acidity and the bond dissociation energies of levoglucosan reveal that the O13− H14 bond is more easily hetero- and homolytically dissociated than the other two bonds, which may be due to not taking part in an intramolecular hydrogen bond.

here, when compared with other results reported in literature. The computational study was extended to the ionization energy, electron affinity, proton affinity, gas-phase basicity, gas-phase acidity, and bond dissociation enthalpies of levoglucosan. The Quantum Theory of Atoms in Molecules was applied to verify the presence of intramolecular hydrogen bonds. Furthermore, a joint differential scanning calorimetry and temperature dynamic FT-IR spectroscopic study was undertaken to study crystalline phase between 298.15 K and the melting temperature of levoglucosan.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed data of the effusion orifices of the Knudsen effusion apparatus, all of the static bomb combustion calorimetry experiments and Knudsen effusion results for levoglucosan, calculated enthalpies using G3 and G3(MP2) computational methods, and Cartesian coordinates of the all the conformations found for levoglucosan. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +351 22 0402 538; fax: +351 22 0402 659; e-mail address: [email protected] (M.D.M.C. Ribeiro da Silva).

CONCLUSION In this work, a comprehensive analysis of the thermochemical properties of levoglucosan was made. Using static bomb combustion calorimetry and Knudsen effusion techniques, it has been possible to derive, respectively, the standard (p° = 0.1 MPa) molar enthalpy of formation of levoglucosan, in the crystalline phase, and the standard molar enthalpy of sublimation, which were combined to calculate the standard molar enthalpy of formation, in the gaseous phase, at the reference temperature of 298.15 K. This value is in good agreement with the value obtained computationally, using G3 and G3(MP2) composite methods. This coherency between the experimental and computational values gives credibility to the combustion calorimetry and Knudsen effusion results, repeated

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to Fundaçaõ para a Ciência e Tecnologia (FCT), Lisbon, Portugal and to FEDER for financial support to Centro ́ de Investigaçaõ em Quimica, University of Porto (strategic project PEst-C/QUI/UI0081/201). I.M.R. and T.L.P.G. thank FCT and the European Social Fund (ESF) under the Community Support Framework (CSF) for the award of the research grant with reference SFRH/BD/61915/2009 and SFRH/BD/62231/2009. G



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Levoglucosan: A Calorimetric, Thermodynamic, Spectroscopic, and

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